Apparatus for deep-well logging



Nov. 25, 1958 s. A. scHERBATsKoY 2,862,106

APPARATUS FOR DEEP-WELL LOGGING 2 Sheets-Sheet 2 Filed July '7, 1955 INVENTOR.

United States APatent 2,862,106 APPARATUS rok DEEP-WELL LOCGING' Serge A. Scherbatskoy, Tul'slag'Okla. Application July 7, 195s, serial No. 520,478 s Claims. (ci. 25d-71) This invention relates to the field of bore-hole logging by radiant energy and is particularly addressed to a radiation detector adapted to be selectively responsive to gamma rays and neutrons of predetermined energy levels.

Radiant-energy bore-hole logging, as carried on in the prior art, involves lowering a radiation detector into a bore hole and recording as a function of depth the intensities of radiation impinging on the detector from the geological formations surrounding the bore hole. Radiation thus impinging on the radiation detector from the formations may be entirely the result ofnatural radioactivity or it may be partially the result of radio-'activity induced by artificial bombardment of the formations with nuclear particles such as` neutrons.` A log produced by detection of neutron-induced radioactivity is known in the art as a neutron-neutronlog or a neutrongamma ray log, depending on whether the radiation detected and recorded consists of scattered neutrons returned to the detectorv or gammatrays `of capture produced in the formations as the result of interactions between neutrons and the atoms in the formations.

The apparatus which forms the subject matter of the present invention is particularly adapted for use in a system of logging in whichtthe formations beingstudied are bombarded with neutrons. Infsuch a logging system, it is theoretically possible, provided a suitable radiation detector is available, to detect the presence of connate water in underground formations, a' highly desirable objective, since it is important to distinguish between connate water and oil.

Connate water invariably contains dissolved sodium chloride in substantial quantities. Chlorine possesses a large neutron-capture cross section, and has the characteristic, upon capturing neutrons, of emittinghighaenrgy gamma rays of capture, having energies inthe neighborhood of 7 m. e. v. Hence the presence of connatewater in the formation surrounding a bore hole can be detected by selectively logging gamma rays from the formations in the 7 m. e. v.e`nergy range.

It is desirable that ysuch neutron-gamma ray logging be accompanied by a neutron neutron log, since -the presence of sodium'and chlorine'in theV surrounding formations Vwill usually be revealed by a 'distinct drop in the number of returned neutrons.` Thus the data pro'- vided by the nt'eutrongamrnaV ray log can be chec'kedbv reference toY the neutron-neutron log. When a marked increase in the intensity of gamma rays in Hthe 7 rn. e. vl region is accompanied by a reduc'tion in the slwlneut'ron` ield, the presence of connte water' in thevsurrounding formations is strongly indicated. i j f v.

The primary object of the pr'esent'invention'is stop'rovide a radiation detector particularly adapted for. hrehole logging in accordance with the system justdescribed.

In furtherance otl that primary object, itE is a further object of the present invention 'to provide a sensitive radiation detector` selectively respnsiveto gamma rays Patented Nov. 25, -1958 of energy levels in the neighborhood of 7 m. e. V., novel means having been employed to make such detector accurate in logging such gamma rays notwithstanding the presence of other radiation fields.

Still another object of the present invention is to provide a neutron detector selectively responsive to slow neutrons having energy levels of a few electron volts, and particularly energy levels in the neighborhood of 1.5 e. v.

Another object of the present invention is to provide, in a radiation detector of the scintillating crystal type, a means for eliminating inaccuracies due to non-uniformity of photo-sensitivity of the cathode of a photomultiplier tube. By the achievement of this object, I have greatly improved the accuracy with which radiations of a desired energy range can be selectively detected and analyzed.

A still further object of my invention is to provide in a radiation detector a novel cooling apparatus operative to maintain the temperature-sensitive portions of my radiation detector at a substantially uniform temperature notwithstanding the high ambient temperatures regularly encountered in deep bore holes. The cooling apparatus referred to is effective, requires attention only at relatively long intervals, and is operative only while the logging apparatus is actually lin use in a bore hole, automatic means being provided for inactivating it at any time the logging apparatus is in a horizontal position, the normal position when not actually in use.

Still another object of my invention is to provide, for a radiation detector, a heat-insulating container for temperature-sensitive elements comprising a double-walled metal vacuum bottle containing finely divided carbon in the evacuated space, a high vacuum and excellent heatinsulating properties being maintained by such structure for long periods despite the slight porosity which is characteristic of thin metal walls.

Y Other objects and advantages of the'present invention will be apparent from the detailed description which follows.

In the appended drawing, Figure .l is a diagrammatic sectional View, taken in the vertical plane, of a radiation detector embodying the principles of Vmy invention, some of the drawing being schematic in character. Fig. 2 is a diagrammatic showing of a typical surface installation for use in connection with the radiation detector of my invention. Figs. 3 and4 are sectional views showing a cooling unit which may form a part of my invention, Fig. 3 showing the unit in the horizontal position which it Will normally occupy when my logging apparatus is being stored, and Fig. 4 showing the same unit in the vertical position which it will normally occupy when the logging apparatus is in actual use in a bore hole.

Referring now to Fig. l, I have indicated therein a bore hole having a metal casing andsurrounding formations 101. My logging unit is carried within a rugged external casing 50, which may be made of steel and which is generally cylindrical in shape, being normally quite elongated and suflciently small in its radial dimension to be received readily within the bore hole.

Near the lowermost portion of the logging unit 50 is a neutron source 20 which Will normally consist of a small vial containing a mixture of radium and beryllium. Such devices are well known in the art as neutron sources.

VDisposed directly above neutron'source 20 is a rela tively massive shield 25 preferably made of tungsten or some heavy alloy such as the product sold commercially under the name Hevimet Shield 25 is intended to shield from the de te'cting apparatus the direct radiations from cylinder, closed oiat'the bottom and provided at the top with a suitable stopper or cork 26. The space between the inner and outerannular walls of bottle 9 is thoroughly evacuated and iilled with finely divided carbon granules 9a; This material, I have found, acts as a gasabsorbing getter and maintains `a very high vacuum for long periods of time despite the slight porosity of the steel walls of bottle 9. Carbon 9a also has the desirable characteristic of absorbing infrared radiation between Vthe walls ofthe bottle and thereby inhibiting further heat transfer from outside the bottle to the protected zone within it, while not interfering with the radiant energy which it is the function of the 'apparatus inside bottle 9 to detect and analyze. p Y

The radiation-detecting apparatus is mounted within a copper inner container 8 which serves more or less as a liner to the vacuum bottle 9 and which serves the dual functions of protecting the equipment from stray highfrequency electromagnetic fields and at the same time maintains the temperaturesensitive elements of the apparatuslat a uniform temperature.

Carried within therupper portion of copper liner 8, in physical contact with its inner wall, I provide a cooling device 28 which will be more extensively described hereinafter in connection with Figs. 3 and 4. For the moment it will be suiiicient to state that cooling device 28 functions, when my logging unit is in a bore hole, to absorb heat from the copper `liner 8 and from the air within the interior of bottle 9, its effect being to maintain substantially uniform the temperature throughout the interior of bottle 9, notwithstanding a substantial rise in the ambient temperature within the bore hole as the logging instrument is lowered therein.

In the lower portion of the copper liner 8 I provide a gamma-ray detector comprising a thallium-activated sodium iodide scintillating crystal 4 and `an associated photomultiplier tube 2. The crystal 4 is surrounded by an aluminumy jacket 5 and an outer shield 6 made of amorphous boron powder dispersed in a plastic binder. The jacket Sand shield 6 play an important part in the operation of my gamma-ray counter, which will be described in detail hereinafter.

Interposed between the face of crystal 4 and photomultiplier 2 is a light-diiusing element 27 made of a suitable ground glass such as the type sold commercially under the name Vycor. Element 27 performs the important function of diifusing light flashes from crystal 4 over a large area of the cathode of the photomultiplier 2. This action averages out the differences in photo-sensitivity of various areas of the cathode and thereby insures that a given flash in crystalv 4 will produce an output pulse of corresponding amplitude. Without the diffusing action of element 27,`the magnitude of any given output pulse will depend not only on the brilliance of the scintillation which produced .it but also on the particular part ofthe cathode on which the ash happens t0 impinge.

An annular shield 7 made ofa highly permeable metal such as Permalloy is mounted so as to surround the photomultiplier unit 2, protecting the highly sensitive apparatus therein from being influenced by stray magnetic fields, particularly those associatedrwith `casing collars. lSuch collars, encountered in boreholes at regular intervals, constitute discontinuities in the diameter of the casing and hence produce discontinuities in the ambient magnetic field within the bore hole.

Externally of casing 50, at the position corresponding to that of the gamma-,ray detector comprising crystal 4, l

provide a thick annular shield 10, fitting tightly around casing 50 and having outer diameter very nearly equal to that of the bore-hole casing 100, being just sufficiently smaller to permit ready raising and lowering of the instrument within the hole. The shield 10 is preferably cast from a mixture of lead and amorphous boron powder. This shield10Y performs very important functions in the 2,862,106 j :M Y

operation of my invention and will be -described in greater detail hereinafter.

Associated with photomultiplier tube 2, I provide an amplifier and power-supply unit 15 which supplies the necessary polarizing voltages for the photomultiplier tube and which greatly ampliiies the power level of the output pulses therefrom. Amplifier 15 is so arranged as to provide positive output pulses responsively to ashes in crystal 4. These output pulses are applied to a gate circuit comprising a high-vacuum dual diode 17. The plates of Ithe diode units in tube 17 are connected together through a polarizing battery 17a which has the effect of biasing one diode plate more negatively than the other. The diode plates are supplied with their principal biasing voltage from the surface via Wire 24, through isolating resistor 24a.

The cathodes of diode unit 17 are connected to the respective terminals -of the primary winding of a pulse transformer 17b, the center tap thereof being grounded.' The secondary winding of pulse transformer 17b has one terminal connected to groundV and the other terminal run to the surface via cable conductor 23.

Mounted within copper liner S, at a position a short distance above the gamma-ray detector which comprises crystal 4, I provide a neutron detector comprising lithium iodide crystal 3 and photomultiplier l. The lithium iodide crystal 3 is activated with europium or tin. f

Interposed between lithium iodide crystal 3 and the photomultiplier tube 1 I provide a diffusing element 27a similar in structure and function tothe diifusing element 27 already described with respect to crystal 4. Y

Closely surrounding the crystal 3, I provide a relatively thck jacket 29 made of nylon, hard rubber,'poly styrene, or other suitable hydrogen-containing material. Jacket 29 acts as a moderator for slowing down neutrons and will be more fully discussed hereinafter.

Outside moderator jacket 29, I provide a shield 12 sur- I rounding the crystal 3. Shield 12 contains samarium or other materials, such as indium or gadolinium, that are characterized by large absorption cross section for very low-energy neutrons. In practice, it is most convenient to prepare .shield 12 by dispersing samarium oxide or other suitable oxides in a plastic binder.

Surrounding photomultiplier 1 I provide an annular Permalloy shield 13, corresponding in structure and function to the shield 7 already described.

Externally of casing 50, but in the position corresponding to that of the crystal'3, I provide a relatively thick annular shield 11 made .of cast nickel.' This shield, like shield 10 already described, plays an important part in the operation of my invention and will be moreV fully described hereinafter.

For cooperation with photomultiplier 1 I provide ampli'- fier and power-supply unit16, which provides the appropriate polarizing voltages for photomultiplier 1 and amplifes the power level ofthe output pulses therefrom. Amplifier 16, like amplifier 15, is arranged to provide positive output pulses. The output of amplifier 16 is applied to' a gate circuit comprising a high-vacuum dual diode 18, having associated with ita polarizing battery 18a and a pulse transformer 18b, connected in a manner corresponding to the arrangement already described with respect to diode 17. Biasing voltage for the diode unit 18 is provided from the surface via wire 22 throughY isolating resistor 22a, output pulses from the pulse transformer 18b being transmitted to the surfacevia'cable conductor 21.

AAs will be understood by persons skilled in theart, the

conductors 21,` 22, 23, and 24 arejincorporated into the long cable 51 by means of which the logging instrument is lowered into the'bore hole. v TheV surface apparatus shown in Fig. 2 does not per se forma part of my invention, but for the sake 'of4 clarity, I shall describe the manner in whichV it is associated with the logging unit of Fig. -1. 1 P

The cable 51 isshown in Fig. 2 as passing over a'pulley agace; 1-0'6 52, which provides a convenient means of determining the quantity of cable that has been'unreled and hence of determining the depth of the logging unit in the bore hole. Pulley 52 is mechanically linked to a reel 54 carrying a sensitized lm 55. As the pulley 52 rotates, reel 54 rotates correspondingly, at some predetermined speed ratio, and hence causes successive parts of the film S5 to be exposed to light beams from the light sources 61 and 62. The light beams reach the film 55 by refiection from mirror galvanometers 63 and 64. The galvanometers, light sources, and lm are of course all mounted in an enclosed housing.

It will be recalled that conductor 21 carries the pulses derived from the neutron detector via the diode unit 18, and conductor 22 supplies biasing voltage for diode 18. At the surface, the conductor 21 is connected to a frequency meter 65, a conventional device which is operative to produce a unidirectional output voltage proportional to ,the repetition rate of the pulses appearing between conductor 21 and ground. The output voltage of frequency meter 65 is applied to the winding of galvanometer 63 and the resulting current causes the mirror to assume angular positions at successive instants of time corresponding to the pulse frequency on conductor 21 at such moments. Thus the apparatus causes a record to be plotted on the film 55 which represents the pulse frequency derived from the neutron detector at each depth as the logging unit traverses the bore hole.

The conductor 22 may be connected at the surface to any suitable source of biasing voltage, such as battery 71 shunted by potentiometer 72, the positive terminal of the battery being grounded and the movable arm of the potentiometer being connected to conductor 22.

A similar arrangement involving frequency meter 75, galvanometer 64, battery 81, and potentiometer S2 is connected at the surface to conductors 23 and 24, for the purpose of providing on film 55 a continuous log, as a function of depth, of the frequency of pulses derived from the gamma-ray detector in the logging unit. The bias on the diode 17 is controlled by adjustment of potentiometer 82, in the manner just described with respect to diode 18.

In the operation of the logging unit of my invention, the gamma-ray detector is normally designed and adjusted to be selectively sensitive to gamma rays of energy in the neighborhood of 7 m. e. v. To that end, the bias on the diode unit 17 will be appropriately adjusted so as to pass pulses of the amplitude experimentally determined to be characteristic of 7 m. e. v. gamma rays.

The operation of the gate circuit comprising diode 17 is as follows: The negative bias provided on wire 24 maintains both diode plates at a negative potential, which effectively suppresses small pulses, since they do notcause either diode to conduct. Very large pulses will drive both diode plates positive and will hence provide simultaneous currents, in opposite direction, in the primary winding of pulse transformer 17b. Thisresults in cancellation, so that again no signal is produced on wire 23.

Pulses in the predetermined critical range, however, will cause one diode to conduct but will not raise the potential of the other diode plate suliiciently to pass current. As a result, current will ow in only one-half of the primary winding of pulse transformer 17b, no cancellation will occur, and an output pulse willV be transmitted to the surface on conductor 23.

From the foregoing, it will be understood that the bias von diode 17, andthe potential of battery 17a, can be adjusted to pass the pulses resulting from interaction of crystal 4 with gamma rays having energies in the neighborhood of 7 m. e. v., which are characteristic of the gamma rays of capture emanated by chlorine.

One source of inaccuracy in such a log, eliminated in my apparatus, arises from Vthe fact that sodium, on being irradiated with thermalv neutrons, emits gamma rays of capture having energies vof aboutI 6.4 rn. e. v. These rays,

6 if allowed to come into being, will interact with the crystal 4 and produce output pulsesr ontconductor 23, since their energy lies within the range of the gamma rays of capture of chlorine.

To prevent generationof such gamma rays of capture in the crystal 4 itself, therefore, I provide-the boron shield 6 and the aluminum shield 5. The boron shield 6 will effectively absorb-neutrons of thermal energy, but it will emit alpha particles as the result of Vsuch neutron absorption. The aluminum shield 5, however, will'absorb the alpha particles without giving rise to any new radiation that willinterfere with the accuracy of the gamma-ray log.

As will be understood from my previous remarks thereon, the accurate spectral analysis of Ygamma rays accomplished by my logging unit is possible only because diffuser 27 substantially eliminatestheerror which would otherwise be introduced by non-uniformity of photosensitivity of the cathode of-photomultiplier tube-2.

I have also found that the failure of previous efforts to achieve accurate spectral analysis of gamma rays of capture produced by neutron interaction with sodium and chlorine atoms in the formations surrounding a bore hole has been in considerable degree due to spurious gamma rays produced by interaction of thermal neutrons with atoms in the various devices which form a part of the logging unit itself. Such devices, which include the steel housing itself, the various metal parts therein included, electron tubes, etc., represent in all a very extensive assortment of elements, which, upon interaction-With neutrons, will emit a broad gamma-ray spectrum including gamma rays in the desired energy zone around 7 m. e. v. In the present invention, I have provided-means to prevent the creation of such spurious gamma rays in the form of the shield 10, made of lead mixed with boron, completely surrounding the gamma-raydetectorr'and situated outside the housing of the logging unit. Not-only'does this lo- -cation of the shield 16 outside the logging unit prevent returning neutrons from entering' the housing and interacting with atoms therewithing-it also has the highly desirable eect of displacing nearly all the' drillingfluid in the zone directly opposite the gamma-ray detector. This further result'is important in procuring -an accurate spectral gamma-ray analysis, because the Vdrilling fluid or mud often contains sodium chloride in significant amounts and other elements which, on interaction with neutrons, will emit gamma rays of capture in and around the 71m. e. v. energy range.

Still another beneficialresult achieved by the external shield 10 is that itl effectively bars from the gamma-ray detector Vthe gamma yrays of 2.2 rn. e. v. energy which origin-ate from interaction of neutrons with hydrogen atoms. While the gate circuit comprising diode 17 is quite effective in preventing the pulses produced by such 2.2 m. e. v. gamma rays from reaching the frequency meter 75, it is nonetheless desirable that those soft gamma rays be trapped out before they reach the crystal 4. Both the -crystal 4 and the photomultiplier tube 2 have a finite recovery time following -a Hash before they are capable of transmitting another flash. Thus the presence of a strong field in the 2.2 m. e. v. spectrum will tie up the crystal and photomultiplier for a substantial fraction of the total time and thereby reduce the probability of detecting gamma rays in the desired 7 m. e. v. range.

The neutron detector comprising crystal 3 and photomultiplier 1 should for most applications be adjusted for optimum response to neutrons of about 1.5 e. v. energy, although for some well-logging conditions it may be desirable to adjust for maximum response to neutrons of higher or lower energy.

The lithium iodide crystal 3, activated with tin or europium, responds to neutrons by reason of interactions between the neutrons and the lithium atoms of mass 6. The reaction may be written Li5+n=iH`3jalpha particle.

The alpha particles are highly ionizing and accordingly make the crystal scintillate. f Y

, The likelihood of the reaction is approximately inversely proportional to the neutron velocity,`with`the result that the crystal per se strongly favors thermal neut(r)or51s of Yextremely low energysubstantially less-than 2 e. v. f-

I have found, however, that neutrons returning from the formations with such low energy levels are not usually satisfactory for logging purposes, because their intensity is very much affectedby such local effects as well diameter and the nature of the drilling lluid. Better logging results are obtained by measuring the intensity of the neutron ux in the neighborhood of 1.5 e. v,

To accomplish that desirableresult, I have provided means for making my neutron detector selectively responsive to such neutronflux. This means'consists primarily of the samarium shield 12, in combination with the nylon moderator 29. The shield 12 effectively eliminates neutrons having less than about 1.5 e. v. energy, and the nylon moderator 29 slows down the neutrons which penetrate shield 12 and reduces their energy to a point where they will produce, proportionately, a large number of nuclear interactions with the Lis atoms in the crystal 3.

Samarium is merely a preferred material for shield 12, other materials having a large capture cross section for low-energy neutrons being usable for that purpose. Examples of such materials are indium and gadolinium. Similarly, the moderator 29 may be formed Vfrom almost any suitable hydrogen-containing material, hard rubber and polystyrene being illustrative examples. Persons skilled in the art will be familiar with a great variety of other materials which will perform effectively as neutron moderators.

The function of Permalloy shield 13 in protecting the photomultiplier 1 from the eiects of changes in the external magnetic elds has already been described.

The shield 11, preferably formed of cast nickel, sharply attenuates gamma rays returning from the formation, while offering no substantial opposition to pass-age of neutrons, and at the same time has the desirable effect of displacing nearly all the drilling uid from the zone around the neutron detector. As with the gamma-ray detector already described, the drilling fluid, if not displaced, can produce local effects impairing seriously the accuracy of the log.

The cooling device 28, which will be described in greater detail presently, may be replaced by some other type of cooling arrangement, such as that described in my copending application Serial No. 395,558. Indeed, in instances where the temperature of the well is not very high, the cooling device 28 can be replaced by a large block of material having high specific heat, such as, for example, a container filled with ice or cold water, or a block of lead or tantalum.

The details of the cooling unit 28 are shown in Figs. 3 and 4. As will be noted from those gures, the container 28 is provided with an intermediate partition 28a having in the center thereof a pinhole aperture 28b. Partition 28a is placed so as to divide the container 28 into two chambers of unequal size, having no communication between them except via the pinhole 2gb. The container 28 is mounted within the copper liner 8 in a position such that the smaller of the two chambers is uppermost when the logging unit is in a vertical position, both chambers being disposed side by side, as shown in Fig. 3, whenfthe logging unit is in a horizontal position.

The two chambers in the unit 28 are somewhat less than half lled with substances which when brought together will undergo an endothermic or heat-absorbing process. VA number of such processes are known to per- 'sons familiar with chemistry. For instance, the temperature of water can be substantially lowered by dissolving in it aV salt, usually nitrate of ammonia. (See in that connection Mechanical Engineers Handbook, edited by L; S. Marks, McGraw-Hill Books, Inc., 4th edition, New York, 1941, page 2138).

When a logging unit is not in actual use in a well, it is normally stored in a horizontal position. Under such conditions, the reacting materials 28e and 28d yare kept separate from one another, and no reaction occurs. When the unit is lowered into a well, however, the liquid material 28e inds itself disposed above the chamber containing the material 28C, so that a slowleakage through pinhole 2817 can take Place, the endothermicreaction proceeding slowly in the chamber containing the material 23d.

When the logging operation is completed, the unit will be restored to its normal horizontal position for storage, and the reaction between materials 28o and 28d will thereupon cease, since in the horizontal position communication through pinhole 28h is cut oi.

This arrangement is greatly superior Vto theuse of a refrigerant such as ice, since ice will melt whether the unit be in use vor not, and hence must be supplied afresh before each logging operation. By proper choice of the size of container 28 and of the reacting materials 28e and 28d, on the other hand, a cooling means can be provided for my logging unit which will require attention only on rare occasions, since the cooling materials are not consumed or caused to change state during periods in which the unit is not actually in use in a logging operation.

While I have in this specification described in considerable detail a typical embodiment of my invention, it is to be understood that that description is intended to be illustrative only. Many changes and variations therein may be made by persons skilled in the art without departing from the spirit of my invention.

I claim:

1. A logging unit adapted to be lowered into a bore hole and being particularly adapted for detection of sodium and chlorine in formations surrounding the bore hole by selective detection of epithermal neutrons and gamma rays of capture returned from such formations responsively to neutron bombardment thereof, comprising, in combination, an elongated metallic housing adapted to be lowered into a bore hole, a neutron source carried within said housing, shield means associated with said neutron source operative to prevent direct radiation from said neutron source from irradiating the interior of said housing but leaving such radiations free to penetrate said housing and to enter formations surrounding said housing, a heat-insulated container within Vsaid housing comprising means for maintaining the temperature within said container no greater than about F. while said logging unit is being operated within a bore hole, and a pair of radiation detectorsl carried within said insulated container, one of said radiation detectors being adapted for selective detection of high-energy gamma rays and comprising a first scintillating crystal, a photomultiplier tube having a photo-sensitive cathode, light-diffusing means for diffusing over a substantial area of said cathode the light from flashes in said crystal, and shielding means surrounding said crystal comprising an outer layer ofmaterial having a large capture cross section for slowA neutrons and characterized by emission of charged particles responsively to Vneutron captures and a metallic inner layer operative to bar passage of such charged particles from said outer layerto said crystal, the other of said radiation detectors being adapted for selective detection of epithermal neutrons and comprising a second scintillating crystal, a second phtomultiplier tube having a photo-sensitive cathode, light-diffusing means operative to diffuse over a substan- `tial area of said last-mentioned cathode the light from flashes in said crystal, a moderator layerrof hydrogencontaining material surrounding said last-mentioned crystal, and other shield means enclosing both said moderator means and said crystal and made from a material characterized by large capture cross`section for thermal neutrons.

2. A logging unit according to claim l wherein said housing is provided with a pair of externalA annular shields, the first of said shields being disposed around the portion of the housing containing said gamma-ray detector and the other being disposed around the portion ofthe housing containing said neutron detector, the first of said annular shields being formed from a mixture containing lead and boron, and the other of said annular shields'being formed from a` material vcharacterized by high` gamma-ray attenuation, both of said shields extending radially outward beyond the normal contour of said housing and being therefore operative to displace additional bore-hole fluid when said logging unit is being used in a tluid-lled bore hole.

3. In a logging unit adapted to be lowered in a bore hole, temperature-regulating means comprising a heatinsulated container and a closed chamber within said container, said chamber having a partition therein operative to divide said chamber into two zones, the orientation of said partition being horizontal when said logging unit is contained within a bore hole and substantially vertical when said logging unit is being stored in a horizontal psition, said partition being imperforate save for a small central aperture, a first material in one of said zones in a quantity somewhat less than half filling it, and a second material in the other of said zones, in a quantity somewhat less than half filling said `second zone, the material in the zone above said partition when said logging unit is in its normal position in a bore hole being in a form permitting it to pass slowly through said central aperture under such conditions, said rst and second materials being characterized, when brought together at a temperature in the neighborhood of 100 F., by undergoing an endothermic reaction.

4. Apparatus according to claim 3 wherein said rst material is water and said second material is nitrate of ammonia.

5. A logging unit adapted to be lowered into a bore hole and being particularly adapted for detection of connate water in formations surrounding the bore hole comprising, in combination, an elongated housing adapted to be lowered into a bore hole, a neutron source carried within said housing, shield means associated with said neutron source operative to prevent direct radiation from 'said neutron source from irradiating the interior of said housing but leaving such radiations free to penetrate said housing and .to bombard the formations surrounding said housing, a heat-insulated container within said housing comprising means for maintaining the temperature Within said container no greater than about 100 F. while said logging unit is being operated within a bore hole, and a radiation-sensitive system carried within said insulated container and comprising a scintillating crystal, a photomultiplier tube having a photo-sensitive cathode, and light-diffusing means for diffusing over a substantial area of said cathode the light from hashes in said crystal, said radiation-sensitive system being selectively responsive toV hard gamma rays of capture having energies in the neighborhood of 7 m. e. v. and being also separately responsive selectively to a second radiation flux returning from said formations responsively to said neutron bombardment, and shielding means surrounding said crystal comprising an outer layer of material having a large capture cross section for slow neutrons and characterized by emission of charged particles responsively to neutron captures and an inner shield layer operative to bar passage of such charged particles from said outer layer to said crystal.

6. A logging unit ladapted to be lowered into a bore hole and being particularly adapted for detection of chlorine in formations surrounding the bore hole by selective detection of slow neutronsand gamma rays of capture returnedlfrom such formations-responsivelyfto neu'- tr'on" bombardmentithereof, comprising, in combination,

an elongated metallic housing adapted to be lowered into a bore hole, a neutronsource carried within'said housing, shield means associated with said neutron source operative toprevent direct radiation from said neutron source from irradiating the'interior of said housing but leaving such radiations free to penetrate said housing and to enter vformations surroundingsaid housing, aheatinsulated container-withinsaid Vhousing comprising means for maintainingthe'A temperature within said container in the neighborhoodof 1009 F. Vwhilesaid logging-unit is exposed to ambient temperatures within a bore hole exceeding F., and radiation-detector means carried at least partially within said insulated container, said detector means comprising a first detection channel adapted for selective detection of gamma rays having energies in the neighborhood of 7 m. e. v. 'and a second detection channel adapted for selective detection of slow neutrons, at least one of said detection channels comprising a scintillating crystal, a photomultiplier tube having a photo-sensitive cathode, and light-diusing means for diifusing over a substantial area of said cathode the light from flashes in said crystal.

7. A logging unit adapted to be lowered into a bore hole and being particularly adapted for detection of connate water in formations surrounding the bore hole by selective detection of slow neutrons and gamma rays of capture returned from such formations responsively to neutron bombardment thereof, comprising, in combination, an elongated metallic housing adapted to be lowered into a bore hole, a neutron'source carried within said housing, shield means associated with said neutron source operative to prevent direct radiation from said neutron source from irradiating the interior of said housing but leaving such radiations free to penetrate said housing and to enter formations surrounding said housing, a heat-insulated container within said housing comprising means for maintaining the temperature within said container no greater than about 100 F. while said logging unit is being operated within a bore hole, and a pair of radiation detectors carried within said insulated container, one of said radiation detectors being adapted for selective detection of gamma rays having energies in the neighborhood of 7 m. e. v. and the second of said detectors being adapted for selective detection of slow neutrons, at least a portion of said rst detector being surrounded by shielding means comprising an outer layer of material having a large capture cross section for slow neutrons and characterized by emission of charged particles responsively to neutron captures and a metallic inner layer operative to bar passage of said charged particles from said outer layer into said shielded portion of said first detector, the second of said radiation ldetectors being at least partially surrounded by a moderator layer of hydrogen-containing material, said moderator layer being at least partially enclosed by an additional shield means made from a material characterized by large capture cross section for thermal neutrons.

8. A logging unit adapted to be lowered into a bore hole and being particularly adapted for identification of sub-surface formations having the characteristic of produring hard gamma rays of capture upon irradiation by neutrons, comprising, in combination, an elongated housing adapted t-o be lowered into a bore hole, a neutron source carried within said housing, shield means associated with said neutron source operative to prevent direct radiation from said neutron source from irradiating the interior of said housing but leaving such radiations free to penetrate said housing and to bombard the formations surrounding said housing, a heat-insulated container within said housing comprising means for maintaining the temperature therewithin in the neighborhood of 100 F. while said unit is being operated within a bore hole, said container having an evacuated zone at least 11 partially lled with a gas-adsorbent material, a radiationsensitive system carried within said housing and being at least partially contained within said insulated container, said system comprising a scintillating crystal, a photomultiplier tube having a photo-sensitive cathode, light-diffusing means for diffusing over a substantial area of said cathode the light from dashes in said crystal, amplifier means fed by the output of said photomultiplier tube, and pulse-height selector means fed by said amplifier means, said system being adjusted to respond selectively to hard gamma rays, and shielding means surounding said crystal comprising an outer layer of material having a large capture cross section for slow neutrons and char- 12 acterized by emission of charged particles responsively to neutron captures and an inner shield layer operative to bar passage of such charged particles from said outer layer to said crystal.

References Cited in the le of this patent UNITED STATES PATENTS 2,522,522 Krasnow Sept. 19, 1950 2,680,201 Scherbatskoy June 1, 1954 2,686,266 Pringle et al Aug. 10, 1954 2,686,268 Martn et al Aug. 10, 1954 2,727,996 Rockwell et al Dec. 20, 1955 

1. A LOGGING UNIT ADAPTED TO BE LOWRED INTO A BORE HOLE AND BEING PARTICULARY ADAPTED FOR DETECTION OF SODIUM AND CHLORINE IN FORMATIONS SURROUNDING THE BORE HOLE BY SELECTIVE DETECTION OF EIPTHERMAL NEUTRONS AND GAMMA RAYS OF CAPTURE RETURNED FROM SUCH FORMATIONS RESPONSEIVELY TO NEUTRON BOMBBARDMENT THEREOF, COMPRISING, IN COMBINATION, AN ELONGATED METALLIC HOUSING ADAPTED TO BE LOWERED INTO A BORE HOLE, A NEUTRON SORUCE CARRIED WITHIN SAID HOUSING, SHIELD MEANS ASSOCIATED WITH SAID NEUTRON SOURCE OPERATIVE TO PREVENT DIRECT RADIATION FROM SAID NEUTRON SOURCE FROM IRRADIATING THE INTERIOR OF SAID HOUSING BUT LEAVING SUCH RADIATIONS FREE TO PENETRATE SAID HOUSING AND TO ENTER FORMATIONS SURROUNDING SAID HOUSING, AND HEAT-INSULATED CONTAINER WITHIN SAID HOUSING COMPRISING MEANS FOR MAINTINING THE TEMPERATURE WITHIN SAID CONTAINER NO GREATER THAN ABOUT 100* F. WHILE SAID LOGGING UNIT IS BEING OPERATED WITHIN A BORE HOLE, AND A PAIR OF RADIATION DETECTORS CARRIED WITHIN SAID INSULATED CONTAINER, ONE OF SAID RADIATION DETECTORS BEING ADAPTED FOR SELECTIVE DETECTION OF HIGH-ENERGY GAMMA RAYS AND COMPRISING A FIRST SCINFILLATING CRYSTAL, A PHOTOMULTIPLIR TUBE HAVING A PHOTO-SENSITIVE CATHODE, LIGHT-DIFFUSING MEANS FOR DIFFUSING OVER A SUBSTANTIAL AREA OF SAID CATHODE THE LIGHT FROM FLASHES IN SAID CRYSTAL, AND SHIELDING MEANS SURROUNDING SAID CRYSTAL COMPRISING AN OUTER LAYER OF MATERIAL HAVING A LARGE CAPTURE CROSS SECTION FOR SLOW NEUTRONS AND CHARACTERIZED BY EMISSION OF CHARGED PARTICLES RESPONSIVELY TO NEUTRON CAPTURES AND A METALLIC INNER LAYER OPERATIVE TO BAR PASSAGE OF SUCH CHARGED PARTICLES FROM SAID OUTER LAYER TO SAID CRYSTAL, THE OTHER OF SAID RADIATION DETECTORS BEING ADAPTED FOR SELECTIVE DETECTION OF EPHITERMAL NEUTRONS AND COMPRISING A SECOND SCINTILLATING CRYSTAL, A SECOND PHOTOMULTIPLIER TUBE HAVING A PHOTO-SENSITIVE CATHODE, LIGHT-DIFFUSING MEANS OPERATIVE TO DIFFUSE OVER A SUBSTANTIAL AREA OF SAID LAST-MENTIONED CATHODE THE LIGHT FROM FLASHES IN SAID CRYSTAL, A MODERATOR LAYER OF HYDROGENCONTAINING MATERIAL SURROUNDING SAID LAST-MENTIONED CRYSTAL, AND OTHER SHIELD MEANS ENCLOSING BOTH SAID MODERATOR MEANS AND SAID CRYSTAL AND MADE FROM A MATERIAL CHARACTERIZED BY LARGE CAPTURE CROSS SECTION FOR THERMAL NEUTRONS. 